Quantum dots (QDs) are semiconductor nanocrystals, generally salts of cations selected from the group consisting of heavy metals including lanthanides, actinides, and transition elements. Those employed for their optical properties typically include transition metals, such as Cd and Zn. Common anions included in the nanocrystalline salt are chalcogenides, Se, S, Te, and O. QDs can be of a core-shell form to modify the QD properties or for passivation of the core with an organic or inorganic coating. The shell typically uniformly surrounds the core. The shell, which is used to passivate the QD, is often a Cd or Zn salt of S or Se or an oxide layer designed to render the QD less toxic.
For optoelectronics applications it is desirable that the QDs have consistent particle size and shape. The particle size of the QD is the predominant determinant of absorption and emission properties for a given type of Quantum dot. However, the quality of the crystal structure, defects, dopants, and impurities can dramatically affect the bandgap and quantum yield. The shell composition and thickness can also affect these properties to various degrees by shifting the bandgap, lowering the quantum yield, or even quenching the emission entirely. The QD size depends on growth conditions, such as, the concentration of reactants, solvent system, temperature, use of surfactants, and reaction time. Traditional methods of semiconductor nanocrystal synthesis are batch processes. For example, the batch method for producing CdSe semiconductor nanocrystals uses a solvent mixture of trioctylphosphine (TOP), tri-n-butylphosphine (TBP), and trioctylphosphine oxide (TOPO) where batch size is limited to production of about several hundred milligrams of TOPO-capped CdSe nanoparticles. It is difficult to scale up batch synthesis due to variabilities in mixing, concentration gradients, and temperature uniformity. Batch synthesis generally produces QDs of relatively uniform size, although batch-to-batch reproducibility is often difficult and achieving the target size is not assured.
To overcome the limitations of the batch process, Barbera-Guille et al., U.S. Pat. No. 6,179,912, teaches a continuous flow process for producing semiconductor nanocrystals. Control of the process at ambient pressure involves selecting appropriate flow rates and temperatures to produce monodispersed QDs of a given size. The continuous flow process uses reservoirs of starting materials that deliver the reagents in series in a first section, then mixes the reagents, nucleates nanocrystals, allows nanocrystal growth, and terminates growth in successive sections of the reactor system. The QDs are often limited in applications by the quantum yield of their emission. In this manner, precise control over the particle size and size dispersity of the QDs is achieved. However, QDs prepared in this manner are capped with the organic ligands such as TOP, and this limits their ability to be used in aqueous systems.
Aqueous batch synthetic routes have been developed, but, typically suffer from long reaction times and the QDs that are produced often display a large number of surface defects that result in relatively poor photoluminescence quantum yields. To overcome these problems hydrothermal techniques have been developed where the synthesis is carried out at high temperatures and pressures in an autoclave. Yet, as with other batch processes, batch to batch reproducibility is often poor and the batch size is limited. Hence there remains a need to prepare consistent QDs and other nanoparticles of a desired size and dispersivity that are amenable to aqueous solutions and do so in a cost effective manner.